Mobile, easy way to add documents to an object?

M-Files Mobile can prefill metadata based on the view where the creation is started, so one option would be to add a view such as "Documents by Case" where Case is used as a grouping level. If the carpenters then create the new documents in these case-specific views they would be automatically linked to the correct case.

The document naming could be automated for instance with Managed Properties or Property Calculator.

This should work. And having a preview of the objects that users selects from the list would be awesome. So user can see details of the object s/he is selecting.

Another option directly adding photos to the object. It's fairly easy to do when you have a view with objects in it.

Of course, thank you for getting me back on track.
The challenge then is that the carpenters would need 2 views - one for cases and one for documents. They need the case view to get a list of their tasks and to register progress as metadata on those tasks. And it would be really hard to explain to them that they must go to different view just to be able to add a picture to a case. Adding pictures directly to the case object is an option, but that creates a whole new set of challenges later in the process when you want to deliver documentation to the customer.
This can be partly overcome by using the "One of" setting on Object Type in the View filter and add Documents next to Case. However, existing documents do not necessarily have all of the properties used to filter and group the cases, so they won't be visible in the combined view. That is not important because the carpenters only need to see the pictures anyway, and for new documents created in this view, properties will automatically be filled with the properties already selected when diving into a specific group. Therefor you can easily create new documents (pictures) in this view.
To make things easier for the users I created a sub view showing only documents in the class Pictures at the grouping level Open Cases \ By department \ By responsible carpenter. It is not quite the same as adding pictures directly on the case, because the carpenter still needs to select the case, but it is close enough to make it an acceptable solution. And of course I have added automatic naming and automatic filling of all other properties to make it easy for the user.

Thanks both of you for your input!